Although a variety of pumping processes have been used in the past to remove gases from a closed container, these processes are similar to each other in that they all employ some physical means to move the gases. For example, a mechanical pump mechanically displaces the gas to remove it; a diffusion or ejector pump entrains gas molecules in a heavy molecular substance such as oil or steam; a cryogenic pump condenses the gas by means of application of a low temperature; a sorbent or getter pump binds the gas on the surface of sorbent molecules; and a blow-down system provides an evacuated chamber which the gas is allowed to fill. While these and other similar pumps perform their intended function (i.e., to raise, transfer or compress fluids), they have disadvantages in some specialized applications, such as pumping a chemical laser, because of their high weight, large size, complex apparatus for implementation, and significant operaing costs. In addition, in certain applications which require a short pump operating time (e.g. 5 to 500 seconds), it is desirable to have a pump whose size scales with the operating time. Most of the pumps mentioned above do not scale with operating time and the pump size is determined solely by the required pumping rate. Furthermore, during the use of these pumps, the pumped gases must be discharged, which could present a problem if these gases are toxic or corrosive.
Such limitations make these pumps unsuitable for some airborne use and space applications. In particular, there is a need for an efficient, compact, light-weight pump which can be used with a chemical laser to remove the flowing reaction product gases and to maintain a pressure of 5 torr or less so that the lasing action can occur. These lasers are used in various applications where a high density of energy is required at a target surface, including such end-item uses as electronic countermeasures devices, target designators, weapons systems and radar systems.
There is one known process which differs from the pumps previously mentioned in that it uses a chemical reaction to remove gases. This type of pump is known as a solid-bed chemical pump and removes gases by a gas-solid chemical reaction. A reactive solid, such as calcium (Ca), is formed into pellets of high surface area. These pellets are then placed in a chamber and the gas to be removed is introduced into the chamber. A chemical reaction occurs between the calcium and the gas, a solid reaction product is formed on the surface of the Ca pellets, and thus the gas is removed. For example, if the gases to be removed are hydrogen fluoride (HF) and nitrogen (N.sub.2), the bulk chemical reactions would be: EQU 2Ca+2HF.fwdarw.CaF.sub.2 +CaH.sub.2 EQU 3Ca+N.sub.2 .fwdarw.Ca.sub.3 N.sub.2
There are, however, several disadvantages in the solid-bed (calcium) chemical pump. The primary disadvantage is that the solid-bed Ca pump is complex and expensive both to fabricate and to operate. In addition, preparation of the Ca pellets used in this type of pump involves a costly, dangerous, and time-consuming process. Furthermore, once the pellets have been prepared, they are subject to violent explosions when carelessly exposed to air. Further difficulties encountered in the operation of a solid-bed Ca pump are that it requires a vacuum seal prior to use and it requires extensive maintenance to replace spent Ca pellets. In addition, during the use of such a pump, dangerous temperature increases can occur for extended run times (e.g., greater than 5 seconds) because of the inability of the Ca pellets to efficiently dissipate the heat generated by the chemical reaction of the gases and Ca. This problem arises because the highly porous structure of the Ca pellets is not an efficient structure for the conduction of heat. However, this pellet structure is necessary in order to have a high surface area of Ca available for chemical reaction. The more the surface area of the Ca is increased by increasing the porosity of the solid Ca, the greater the heat transport problem becomes. When the heat of reaction is not dissipated, the Ca pellets themselves attain excessive temperatures and some reaction products initially formed are decomposed by the heat (i.e., the chemical reaction is reversed), and the solid-bed Ca pump does not function. This limitation makes the solid-bed Ca pump of limited application for a chemical laser system that might require pumping for 5 to 500 seconds of continuous operation.